1. Field of the Invention
The present invention relates to methods and devices for suitably vaporizing, mixing, and delivering liquid fuels or liquified gases for use in combustion devices.
2. Background of the Technology
Combustion devices, such as gas turbines used for power generation, are typically fueled by natural gas (e.g., compressed natural gas or CNG). Typically, natural gas consists of approximately 90–98% by volume methane (CH4), although some gases with as little as 82% methane have been characterized as natural gas. Other than methane, natural gas may include CO2, O2, N2 and higher hydrocarbon gases, such as C2 (ethane, ethylene, acetylene), C3 (propane), C4 (butane), and C5 (pentane).
Recent advances in the design of the combustion systems for gas turbine engines have resulted in substantial improvements in exhaust emissions during operation on natural gas through the use of lean, premixed combustion. In this combustion mode, natural gas is premixed with combustion air prior to arrival at the flame front. This lean mixture of natural gas and air burns at a lower temperature than conventional diffusion flame combustors, thereby producing lower levels of pollutants, including oxides of nitrogen (NOx) in the exhaust stream. By way of example, the maximum allowable NOx levels for diffusion flame combustors is typically 42 ppm @15% O2, while the maximum allowable NOx levels for a lean, premixed combustion gas turbine is now typically 15 ppm@15% O2. The 42 ppm NOx level for diffusion flame combustors generally can only be achieved through the addition of large amounts of steam or water into the combustor to reduce the flame temperature.
Attempts have been made to operate lean, premixed combustion devices with alternate, higher hydrocarbon liquid fuels such as oil and diesel fuel and higher hydrocarbon fuel gases such as propane (C3), and butane (C4). As used herein, “higher hydrocarbon fuel” refers to a fuel wherein at least 50 weight percent of the hydrocarbon molecules of the fuel have at least two carbon atoms. Unfortunately, these combustion devices cannot be readily operated in a lean, premixed, prevaporized (LPP) combustion mode when using the alternate fuels. In order to generate a lean, premixed, prevaporized flame using liquid fuels or liquified gases (as used herein, the term “liquid fuel” should be understood to include fuels that are normally in a liquid state at room temperature and atmospheric pressure, as well as gases that have been liquified by cooling and/or pressurizing), the liquids must first be evaporated into a carrier gas (normally air) to create a fuel gas (i.e. a fuel vapor/air mixture) which then may be mixed with additional combustion air prior to arrival at the flame front. However, a phenomenon known as auto-ignition can occur with such vaporized liquid fuel/liquified gas and air mixtures. Auto-ignition is the spontaneous ignition of the fuel prior to the desired flame location in the combustion device. This premature ignition can occur, for example, as a result of normal, premature, or other heating of the fuel that can occur as the fuel is fed to the combustion device. Auto-ignition results in decreased efficiency and damage to the combustion device, shortening the useful life of the combustion device and/or causing an increase in unwanted emissions.
Various attempts have been made to curtail auto-ignition of higher hydrocarbon liquid fuels in such lean, premixed combustion devices, but none of them have proven entirely successful. As a result, “dual fuel” combustion devices, such as gas turbines, capable of operating with both natural gas and higher hydrocarbon liquid fuels typically operate in an lean, premixed mode when used with natural gas and in a diffusion mode when used with higher hydrocarbon liquid fuels. Combusting the liquid fuels in the diffusion mode is undesirable as it increases NOx and other emissions as compared to natural gas combusted in the lean, premixed mode.
Another issue that has recently become of increased importance is a problem associated with the use of liquified natural gas. A recent shortage in the domestic natural gas supply has made the importation of liquified natural gas more common. When liquified natural gas is shipped, typically via tanker, the higher hydrocarbon gases have a higher boiling point. When the liquid natural gas is re-vaporized for use as a gaseous fuel, the last portion of liquified natural gas removed from the storage container contains a higher percentage of higher hydrocarbon fuels. Due to the aforementioned auto-ignition problem, this portion of the liquified natural gas cannot be used in many existing lean, premixed natural gas combustors.
Combustion devices similar to those used with natural gas are also used on boilers, incinerators, and turbine engines, and other combustion engines, including applications other than power generation, such as for propulsion for naval ships. Problems with use of turbine engines for naval ships include the large amount of storage space typically required for conventional compressed gas fuel and high emissions that result from alternative fuel use in conventional turbine engines. The emissions can both violate environmental requirements and present a security hazard by, for example, producing visible emissions that reveal the position of the vessel.
There remains an unmet need for combustion devices such as turbine engines and other combustion devices that can be operated with both natural gas and higher hydrocarbon liquid fuels in a lean, premixed, pre-vaporized mode. A satisfactory dual fuel option for such combustion devices would allow, for example, cost and fuel flexibility for applications such as power generation and others.